I don't have the exact answer but I've seen the snowballs spawning a few times in my island as I logged in and out, hoping to catch dung beetles: They absolutely spawned always in pairs, not more than two screens away from each other, sometimes over a flower or pumpkin. Only three spots seem big enough for them to appear, and the smallest of them is clearly smaller than the plaza. Not sure why they aren't spawning for you, Tavernade
^ Took a picture to see how much space is needed for them to spawn.
Thanks you two! I took out a bigger flower bed and got them right away!